I think this is an area that is the least understood, and needs more clarification, especially as it pertains to anyone keeping or intending to keep clams, corals or pacific host anemones. Why is lighting so important? That's because these animals derive much or most of their nourishment from symbiotic algae cells imbedded in their tissues called zooxanthellae. These algae cells are nourished by the byproducts of respiration and digestion, and in turn provide nourishment through photosynthesis, to their host animal. Each one derives and provides essential nutrients to perpetuate the relationship, without which, both perish.

Its very important to imitate, as much as possible, the same conditions in which these animals have been, and evolved in, for millions of years.

At this juncture, I must combine the theoretical aspects of Physics as it pertains to natural sunlight and artificial light for our aquariums. In my discussion, I want to concentrate first, on what is happening to light in respect to coral reefs, and secondly, artificial lighting. All data is taken from, by and large, measurements of unobstructed sunlight at the equator, on the ocean's surface, at high noon, as published in the scientific literature.

There are three critical areas of lighting that need to be understood, in order to fully appreciate the problems facing the establishment of a Reef Aquarium, in terms of what sunlight is, and how it is utilized by photosynthetically dependent animals.

These are:

SPECTRUM - That portion of visible light that is important for photosynthesis, pertaining to the specific needs of marine animals and their zooxanthellae.

LUX - The intensity of sunlight needed to penetrate ocean water for photosynthesis to occur.

PAR - Photosynthetically Active Radiation - That part of the Sun's spectrum that is utilize by all life forms whose cells contain chlorophyll, and with additional nutrients, converts sunlight into nourishment. Our discussion will primarily focus on those microscopic algae cells known as zooxanthellae, living in the tissues of most corals, clams, and anemones, and again, whose survival depends on adequate lighting.

At this juncture an understanding of the meaning of the KELVIN scale is needed. Its simply easier to explain Kelvin in terms of its applications, rather then its exact scientific implications. In the simplest of terms, Kelvin refers to a scale where the theoretical limits of the absolute temperature of molecular energy, starting with zero activity (0� Kelvin) to infinity, has been devised. Its scale is really of more use to astronomers, in calculating distances and time/space continuums (the ages) of stars.

As a way of a simpler explanation, imagine taking a piece of iron (known in the literature as that theoretical 'black body' used to extrapolate data) heated up to a certain Kelvin temperature, and then measuring the spectrum of light waves emanating from that heated body. This is how the Kelvin temperature of the Sun is calculated, theorized to be made of iron, and the spectral content of the light emitted is equal to heating our black body to 5800� on the Kelvin scale. That's always our reference point.

There are two aspects in the way light is measured. The first is light waves, which are measured in units called nanometers, a billionth of a meter, the visible spectrum ranging from 700nm, corresponding to the 'color' Red, to wavelengths of 400nm corresponding to the 'color' Violet. The other is intensity, in units called photons. This means that light waves striking our eyes, with enough intensity, produces electrical stimulations in our cone cells, specialized receptors in our retinas for color, that send electrical impulses to our brain where images are formed, and we agree upon, to call these "Red" or "Violet" or any of the other 'standard' colors. So much for the Psychology of Perception.

Now something happens to sunlight, as it penetrates the ocean's water. The water filters out or absorbes most of the wavelengths, corresponding to longer wavelengths of 700 nm. (red) to approx 600nm (yellow). This means that within the first 3 feet of the oceans depth, almost 80% of wavelengths of light corresponding to Red, Orange, and Yellow are absorbed by the water and only those wavelenghs shorter then 600nm penetrate the water. This wavelength (600nm) represents the dividing line between Yellow and Green.

What does this have to do with Kelvin? Back to our analysis of that theoretical 'black body'. Now, spectral analysis of light, emitted from the Sun (5800�K) shows that it gives off equal wavelengths of light, of the visible spectrum, so combined, all the individual wavelengths, appears to us as 'white'.

However, direct sunlight looks more yellow to us, or actually yellow/orange, because our eyes have a greater sensitivity in this region for color. This should not be confused with perceived intensity. In terms of the relationship of perceived intensity of color, we are the most responsive to green.

To further clarify this area, if we divide all the colors of sunlight through a prism, the color 'green' appears more intense then the others. However, in terms of our eyes sensitivity threshold to color, which is, at what point do we recognize color, we are the most sensitive to yellow/orange, the last color our eyes 'sees', as light intensity diminishes. That's why, for the most part, the color of the paint on roadways, especially the center lines, is a yellow/orange.

Since lamps are designated by their Kelvin temperatures, their perceived brightness cannot be judged by our eyes alone, because our perception of brightness is not uniform, just as wattages are also poor indicators of brightness. In order to make the right choices, we need to know two specifications, Kelvin temperature, and Lumen intensity. Now lets say, we heat that body of iron to 6500�K, the corresponding wavelengths of emitted light, would shift towards the shorter end, and the light would start appearing a little more 'blue' to us. In fact, the higher we heat that 'black body', the fewer wavelengths of light corresponding to the longer (Red, Orange, Yellow and Green) wavelengths are being emitted, and the more wavelengths of light corresponding to the Blue and Violet spectrums, the shorter wavelengths, are measured, and do we see.

So the Kelvin rating of a Lamp refers to its equivalent spectrum spread of a black body heated to that Kelvin temperature. The lower the Kelvin temperature rating, the more it emits wavelengths corresponding to colors we call red, orange, yellow and green. The higher the Kelvin designation, the more intense are wavelengths corresponding to the colors of blue and violet.

That doesn't mean that a lamp rated at 10,000� Kelvin is not giving off any red, it only means that the predominant wavelengths of light being emitted, that our eyes 'see' corresponds to the color 'blue'.

Now lets put this in perspective, in terms of lighting our aquarium. We know that most wavelenghts of sunlight are filtered out by the water, and, as far as photosynthetic dependent animals are concerned, only those wavelenghts of light between 400 nms and 600 nms are being utilized by the algae cells in their bodies.

Now, the lower down in depth these animals reside, the greater filtering effect of the water, where only the shortest wavelengths are able to penetrate, and since our sensitivity to light diminshes, the less brightness we perceive. This is not to say that lower light requiring animals need lower light intensities, but only that the useable spectrum they need, 'appears' less intense to us. That's why 'blue' lamps appear not as bright to us as 'white' lamps.

Analysis of Kelvin temps in relation to the ocean's depth has shown that for the most part, the most highly photosynthetic dependent animals, such as Clams, SPS Corals and Pacific host Anemones, live within the first meter of the ocean. They require light closer to the full spectral content of sunlight, which is a Kelvin temperature rating of 5800� to 6500�, in order to benefit by the wavelengths needed, for photosynthesis to take place. At the lowest ends of the scale, effective Kelvin temps of 12,000� for our 'lower light' corals, corresponds to the shortest wavelenghts of light that can be utilized by photosynthetic animals.

Lamps with Kelvin designations higher then this, provide substantially less light in the 'optimum' photosynthetic zone. Ideally, lamps between 5800�K and 12,000�K produce substantially 98% of the requirements of all photosynthetic dependent ocean animals and plants. Using lamps of higher Kelvin temperatures seem to only benefit the user, and not for the benefit of photosynthesis.

Sunlight striking the ocean's surface, is scientifically measured in units of light intensity called Lux. However, the popular designations for our lamps is in Lumens. Sunlight at the equator at high noon, is equal to the amount of light emitted by 10,000 standard candles (candle-power or lumens) over one square foot, or 100,000 Lux per square meter (the scientific standard of illumination).

Sunlight loses about 20% of its energy in the first meter, where the most demanding photosynthetic animals live, SPS Corals, Pacific host Anemones and many species of Clams. In the deeper regions of the Reef, where lower light requiring animals are found, light intensities drop off as much as 80%.

Because of the very real problems in simulating sunlight in an aquarium, for all practical purposes, I will be using minimal requirements. This simply means that, to minimally simulate optimal sunlight at high noon, at the depth of one meter, we need approx 10,000 lumens of light energy for every square foot of our aquarium's surface area, radiating into our aquarium. For our purposes, I will be using a standard aquarium designated at 100 gals, which has approx 8 sq feet of surface area. This tank then requires a minimum of 80,000 lumens (10,000 x 8) to approximate sunlight within the first meter in depth. Minimally, that's 80,000 lumens directed into the aquarium, of net energy. For the benefit of calculations, I have taken the liberty of rounding off numbers, as all values are approximations, at best, and I also take further liberties, since our standard aquarium is less then one meter deep.

One of the main problems about making comparisons between sunlight and lighting our tanks, is that sunlight represents a point source of light. Unfortunately, the same cannot be said about the lamps we use to light our tanks. As far as fluorescent lamps are concerned, only about 25% of the light output is directed into our tanks, and 75% radiates in other directions, and goes elsewhere. MH lighting is only slightly better in this respect. To account for fixture design and reflection redirection, I would venture that, on an average only about 50% of the light goes into the tank, especially with fluorescent lighting. That means that 50% of your lamps lumens are being directed to our target area, and 50% is being lost through back and side reflections, etc. Now you still need that 80,000 lumens, but you need to double your lamps, so they produce 160,000 lumens of light intensity.

Now, what does that really mean to us, and how many lumens do our lamps put out. Just for an example, a GE� 40W ultra daylight lamp with a Kelvin designation of 6500�, emitts 3,050 lumens of energy. Back to our 100 gal aquarium. With 8 sq feet of area to illuminate, trying to equal sunlight at the equator, at the depth of one meter, 10,000 lumens for every square foot, times 8 square feet, equals 80,000 lumens. Now lets take that 40W GE lamp, rated at 3,050 lumens. We can say that even with specially constructed reflective material only half of its light is being directed at our tank, so that's only 1,525 lumens. Now we need 80,000 lumens, so, that means we need (approx) 52 - 40W GE Lamps. Now that's 2,080 watts of power for our 100 gal tank, right, so that's 20.8 watts per gallon. Lets use another example, of VHO lamps. Typically 110W - VHO 6500�K lamps have approx. 7,250 lumens of light emissions, so you would need 22 110W VHO's (with 50% efficiency, 3,625 � into 80,000) for our needs, which is 2420 watts, or 24 watts per gallon. As you can see, "rules of thumbs" don't work for fluorescent lighting, where the recommendation is 10 to 12 watts per gallon, plus the problem is logistical, in the placement of 22 - 110W VHO's over a 100 gal. tank. Fluorescent lighting is only recommended where the demands for photosynthesis are at the least. Typically, 4 -110W - 6500�K VHO's or 96W - PC's would work with lower light demanding corals, such as Mushrooms and Polyps, which require 4 to 5 watts per gallon for our 100 gal aquarium.

For the most part when dealing with highly photosynthetically dependent animals, two or more 400W 6500�K MH's with typically intensities of about 34,000 lumens, mounted in specially constructed enclosures where highly reflective materials are used, which re-direct 80% of the reflective light, can only meet these demands. Just for the sake of calculations, our MH has an average useful intensity of 34,000 lumens, but even under the best of conditions loses 20% of this due to reflections, so its net intensity is 27,000 lumens. Now back to our requirements of 80,000 lumens. Our 100 gal aquarium requires about 3 - 400W 6500�K MH's to meet our needs. Converting these figures to watts per gallon, we can see that's about 12 watts per gallon, so our 'rule of thumb' does work!

Another area of great importance, especially to understanding sunlight, is PAR, Photosynthetically Active Radiation, that portion of visible energy (sunlight) striking the earths surface, important for photosynthesis, the basis of all life on earth, covering the visible spectrum of 400nm to 700nm.

Besides wavelength, sunlight is also measured in terms of radiated energy called photons, and these are in units called Einsteins. Because radiated energy covers an entire daylight period, this unit is rather large, and for our purposes of discussion, typically measurements are made in millionths of an Einstein, denoted as microEinsteins per second. Because energy is also area related, photonic energy uses nomenclature that specifies the area covered as well as the time period of radiation, as microEinsteins per meter squared per second (uE/m�/sec).

Typically, the amount of radiated energy of the sun on a cloudless day, at the equator is 2,000 micro Einsteins, per square meter of area per second. (uE/m�/sec), at the water's surface. Suffice is it to say, that the definition of this unit is beyond the scope of further explanations, and is in the realm of theoretical physics. We only need to be concerned with these energy designations as a standard, to base our lighting demands.

Now this energy level is not constant, and varies with the time of day, just the same way as Kelvin temperatures also vary, because of the changing angles in which sunlight is striking the earth, as well as attenuation of sunlight by clouds.

Because lamps do not give us data as to their PAR values, we are dependent on using lumens in determining intensities, based on the photosynthetic needs of our animals being kept. Lumens and Kelvin temperatures are what's really important.

For highly photosynthetic dependent animals as, SPS corals, pacific host anemones, and most clams, which are found in the shallowest parts of the reefs, full spectrum lamps in Kelvin temps in the 5800� to 6500� range work the best, and minimum rated consumption of 10 to 12 watts/gal using MH lighting. In this instance, only MH lighting really meets these requirements. For a 100 gal tank, where highly photosynthetic invertebrates are desired, independent tests have shown that 3 - 400W MH 6500�K lamps from Iwasaki� or 3 -10,000� K lamps from HQI� are recommended, with actinic NO's or VHO's for supplementation and sunrise/sunset simulation.

Medium light corals, and other animals, benefit from slightly higher Kelvin temps, ranging from 6500� to 10,000�, and minimum lamp wattages of 7 to 8 watts/gal. are necessary. 400W or 250W MH's supplemented with VHO or NO actinics are recommended. Examples of medium light corals are bubble corals, open brain corals, cabbage and leather corals. What distinguishes these corals from lower light corals is the fact that the corals have pastel colors, whereas the lower light corals are primaily brown or orange in color.

Although corals found at greater depths, such as the many and varied species of mushrooms, as well as many polyps, benefit more from Kelvin temps corresponding to the higher designations of 10,000� to 12,000�, and a minimum of 4 to 5 watts/gal., its also been shown that they too benefit from full spectrum lighting available from many 6500�K lamps. In this instance one could use either 110W VHO's or 96W - PC's for lighting. Their brown pigmentation indicates a greater concentration of zooxanthellae, and they seem to require less light, then other more delicately colored corals. Even with lower light corals, it is still be difficult to achieve this intensity, without some MH lighting. Observations have shown that when various species of the higher light requiring corals are subjected to lower light levels, they compensate by increasing their zooxanthellae algae cells and become browner, as zooxanthellae are basically brown in color. These cells intensify in number to take advantage of the decreased lighting, and cause the once delicately and often brilliantly colored corals to turn drab, a direct result of inadequate lighting intensity. There is a delicate balance between these two relationships, corals and their zooxanthellae. As the corals starve, they shrink, providing less nourishment to the zooxanthellae, who in turn start dying off. Once this starts, it seldom can be reversed. Adequate lighting is the key.

One thing is known, photosynthetic animals will adapt to lower light conditions then their more 'natural' requirements. However, especially with animals needing those higher light intensities, their lives will be significantly shorter, and their appearance will suffer. In the case of pacific host anemones, their life spans have been calculated to exceed one hundred years and coral colonies, several hundred years!

As I have outlined, and has repeatedly been stated in the literature, by many respected authors, and contrary to many expressed 'popular' opinions and practices, for the most part, invertebrate aquariums, especially those with highly photosynthetic corals, clams, and anemones, are greatly underlit, and many failures can be attributed to this one factor, more then anything else. This may not be apparent immediately, and these animals seem to survive, but, as the zooxanthellae struggle to maintain photosynthesis, their numbers do dwindle, and unexpectantly, and sometimes quite suddenly, the coral colony, clam or anemone will just die, because the balance between zooxanthellae's needs and their hosts metabolism is very fragile.

It behooves the conscientious aquarist to first, research the needs of their contemplated purchases, and then tailor the aquarium to meet those needs. If that is not feasible or possible, then one should forego these purchases, as more suitable alternatives are readily available. IMVHO, the purpose of this hobby should be to provide protection and attempt to preserve the lives of our charges, rather then the collection of animals for our own amusement, without regard to their special needs or requirements.